This invention relates generally to strain gauge based sensors, and more particularly, to strain gauge based sensors having diaphragms with a high thickness to diameter ratio, such as would be utilized in relatively high pressure applications.
It is well known in the art to utilize strain gauge based sensors to measure pressure. A particular type of strain gauge based pressure sensor, as an example and for purposes of describing a prior art type of such sensor, can be threaded into the fuel rail of a diesel engine. Such sensor has a general appearance of a boltxe2x80x94a threaded shaft portion extending downwardly from a hex shaped head portion, by which shaft the sensor can be threaded into the aforementioned fuel rail. However, unlike the bolt which it resembles, the threaded shaft of the sensor has a cylindrical bore which extends upwardly through the hex shaped head and into a generally cylindrical portion projecting above the head, which is commonly referred to as the xe2x80x9cdiaphragm framexe2x80x9d of the sensor. The bore extends up into, but not through, the diaphragm frame, leaving a certain thickness of material above the bore which is referred to as the xe2x80x9cactive diaphragm.xe2x80x9d Although there is a corner radius (r) blending the bore into the underside of the active diaphragm, the diameter of the active diaphragm is generally considered equal to the diameter of the bore. Conventionally, four relatively large individual strain gauges are affixed to the top surface of the active diaphragm, and perhaps partially on the diaphragm frame, for transforming the pressure induced deflection of the diaphragm into electrical output representative of the magnitude of the pressure acting on the underside of the active diaphragm.
The primary design factors in constructing such strain gauge based sensors are the diaphragm thickness (xe2x80x9ctxe2x80x9d); the active diaphragm diameter (xe2x80x9cDxe2x80x9d); and the diaphragm frame diameter (xe2x80x9cdxe2x80x9d). The thickness of the active diaphragm has a very significant effect on the static accuracy of the sensor. Thinner active diaphragms generally provide better static accuracy, for reasons which will be explained in more detail hereinafter. The static accuracy of strain gauge based sensors is typically discussed in terms of the xe2x80x9clinearityxe2x80x9d of their output and refers to the linearity and hysteresis of the sensor. Strain gauge based sensors typcially use four (or two) strain gauges connected into a wheatstone bridge configuration. When excited by a constant voltage (or current), the bridge provides a voltage output (e.g. in the millivolt range) which is directly related to applied pressure. The strain gauges are typically located on the diaphragm surface such that the resistance of two (or one) of them increases with applied pressure, while the resistance of the other two (or one) decreases. Optimal linearity is achieved when the magnitude of these resistance changes are equal. A problem with static accuracy arises when designing strain gauge based sensors which must be utilized in high pressure applications. In such applications, the active diaphragm of conventional strain gauge based sensors must be constructed with a relatively high thickness (xe2x80x9ctxe2x80x9d) to diameter (xe2x80x9cDxe2x80x9d) ratio (xe2x80x9ct/Dxe2x80x9d) in order to structurally withstand the high pressures. This is disadvantageous because an active diaphragm having a relatively high t/D ratio generally has a lower degree of accuracy. The short explanation for this, without digressing into a discussion of theoretical equations, is that simple diaphragm theory holds true when the thickness to diameter ratio (t/D) of the active diaphragm is about 0.15 or less. According to simple diaphragm theory, when the t/D ratio is 0.15 or less the deflection of the active diaphragm is almost entirely dominated by bending stresses, with shear stresses considered negligible or insignificant such that radial stress essentially equals bending stress. However, as the t/D ratio increases, so do the shear stresses, which can no longer be considered insignificant. Consequently, simple diaphragm theory will not apply. Moreover, the shape of the deflection curve is altered by the increased shear stresses which also results in degrading the linearity of the output of the wheatstone bridge, and hence the accuracy of the sensor.
In the prior art, low pressure sensors are known to be susceptible to output shifts/drifts as a result of stress from mounting the sensor being induced in the active diaphragm. The mounting stress can be induced as a result of over-tightening the sensor into a threaded opening. As a means of relieving mounting-induced stress, some prior art low pressure strain gauge sensors (e.g. 2500 psi and below pressure sensors) are known to have a groove provided in the diaphragm frame of the sensor such that torque induced stress transferred to the active diaphragm or the diaphragm frame is minimized. However, the groove in the diaphragm frame does not improve the linearity of the output of these low pressure strain gauges. In such low pressure applications, a high t/D ratio is unnecessary. Thus the t/D ratio of such low pressure sensors is designed at 0.15 or less, whereby the linearity of the output is already optimal. In the absence of a high t/D ratio active diaphragm, such as required in high pressure applications, the groove is of negligible or no value in low pressure strain gauge sensors except for the aforementioned purpose of stress relief.
Accordingly, there is a need for a strain gauge based sensor which provides improved linearity output for high pressure sensors having active diaphragms with a high t/D ratio.
A strain gauge based sensor for use in relatively high pressure applications, having an active diaphragm with a high t/D ratio, is provided with an annular groove in the outer periphery of the diaphragm frame of the sensor which results in improved linearity of the output of the strain gauges. The annular groove preferably is provided entirely around the periphery of the diaphragm frame, however it can also be that only a portion, or portions, of the periphery are provided with an annular groove. This would also result in some degree of improved linearity. The annular groove in the periphery of the diaphragm frame alters the strain field on the surface of the high t/D active diaphragm, causing the radial stresses to be primarily bending stresses and minimizing shear stresses. As a result, the high t/D active diaphragm sensor behaves more like a sensor having a diaphragm with a t/D of 0.15 or less, wherein active diaphragm deflection is almost entirely dominated by bending stresses and shear stress are insignificant. The annular groove is provided in the diaphragm frame at a location near enough to the top surface such that the portion of the diaphragm frame above the annular groove is able to flex slightly under the pressure applied to the sensor. However, the depth and width of the frame groove must also be designed such that the sensor can withstand the high pressure or load to which it is subjected. The slight flexing of the portion of the diaphragm frame above the annular groove alters the strain field, thus inducing the increased bending stresses and minimizing the shear stresses, which ultimately results in the improved linearity of the output of the strain gauges. The diaphragm frame, active diaphragm and bore may also have a non-cylindrical cross section shape, in which case a ratio in terms of thickness to surface area can be used instead of t/D. This invention may also be used with all types of strain gauges, including individual strain gauges, integrated half bridge gauges, or integrated full bridge strain gauges, to name a few. Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawings figures of certain embodiments thereof.